coelom coe·lom or ce·lom or coe·lome (sē'ləm)n. The cavity formed by the splitting of the embryonic mesoderm into two layers; in mammals it then forms into the peritoneal, pleural, and pericardial cavities.

(sē'ləm) The body cavity that forms from the mesoderm during the embryonic development of more complex animals. The coelom suspends the gut in fluid in the middle of the body, protecting it from gravity and allowing great increases in body size. The presence or absence of a coelom is important for the classification of animal phyla. See more at deuterostome, protostome.